ABSTRACT Gut microbial communities influence multiple facets of human health, impacting common disorders such as obesity, autoimmune diseases, and allergies. These discoveries were enabled by the application of metagenomic sequencing and mass spectrometric techniques to human fecal samples, leading to remarkable insights into the interplay between gut microbial communities and human health. However, a major barrier to translating our growing knowledge about the gut microbiota and its functions to the patient-care setting is the difficulty in obtaining, storing, processing and analyzing fecal samples. Volatile organic compounds (VOCs) are present in exhaled breath and are produced by many microbes. By coupling a new method for analyzing the VOC composition of mouse breath samples with our gnotobiotic model, we find that breath VOC patterns reflect the gut microbial communities. Based on this preliminary data, our central hypothesis is that many VOCs observed in breath are produced by the gut microbiota and thus represent biomarkers of gut microbial community configuration and function. We will test our central hypothesis through the following specific aims: (1) Evaluate the extent to which gut microbial communities shape breath VOC profiles; and (2) Characterize human gut microbes that generate VOCs and demonstrate that microbe-produced VOCs are present in the breath of gnotobiotic mice. Our investigations will be significant, because we will greatly advance our fundamental understanding of the gut microbial contribution to breath volatile composition, and we will begin to identify specific, individual volatile-microbe correlations. Our work will therefore launch a new paradigm towards novel, non-invasive ?breathalyzer? diagnostics to interrogate gut microbial dysbiosis.